Re: [PATCH V1 01/16] x86/ioreq: Prepare IOREQ feature for making it common

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.09.2020 16:43, Oleksandr wrote:
> On 21.09.20 16:29, Jan Beulich wrote:
>> On 21.09.2020 14:47, Oleksandr wrote:
>>> On 21.09.20 15:31, Jan Beulich wrote:
>>>> On 21.09.2020 14:22, Oleksandr wrote:
>>>>> On 14.09.20 16:52, Jan Beulich wrote:
>>>>>> On 10.09.2020 22:21, Oleksandr Tyshchenko wrote:
>>>>>>> @@ -1215,8 +1233,7 @@ void hvm_destroy_all_ioreq_servers(struct domain 
>>>>>>> *d)
>>>>>>>         struct hvm_ioreq_server *s;
>>>>>>>         unsigned int id;
>>>>>>>     
>>>>>>> -    if ( !relocate_portio_handler(d, 0xcf8, 0xcf8, 4) )
>>>>>>> -        return;
>>>>>>> +    arch_hvm_ioreq_destroy(d);
>>>>>> So the call to relocate_portio_handler() simply goes away. No
>>>>>> replacement, no explanation?
>>>>> As I understand from the review comment on that for the RFC patch, there
>>>>> is no
>>>>> a lot of point of keeping this. Indeed, looking at the code I got the
>>>>> same opinion.
>>>>> I should have added an explanation in the commit description at least.
>>>>> Or shall I return the call back?
>>>> If there's a reason to drop it (which I can't see, but I also
>>>> don't recall seeing the discussion you're mentioning), then doing
>>>> so should be a separate patch with suitable reasoning. In the
>>>> patch here you definitely should only transform what's already
>>>> there.
>>> Sounds reasonable. Please see the comment below
>>> relocate_portio_handler() here:
>>> https://www.mail-archive.com/xen-devel@xxxxxxxxxxxxxxxxxxxx/msg78512.html
>>>
>>> However, I might interpret the request incorrectly.
>> I'm afraid you do: The way you've coded it the function was a no-op.
>> But that's because you broke the caller by not bailing from
>> hvm_destroy_all_ioreq_servers() if relocate_portio_handler() returned
>> false. IOW you did assume that moving the "return" statement into an
>> inline function would have an effect on its caller(s). For questions
>> like this one it also often helps to look at the commit introducing
>> the construct in question (b3344bb1cae0 in this case): Chances are
>> that the description helps, albeit I agree there are many cases
>> (particularly the farther you get into distant past) where it isn't
>> of much help.
> 
> 
> Hmm, now it's clear to me what I did wrong. By calling 
> relocate_portio_handler() here we don't really want to relocate 
> something, we just use it as a flag to indicate whether we need to 
> actually release IOREQ resources down the 
> hvm_destroy_all_ioreq_servers(). Thank you for the explanation, it 
> wasn't obvious to me at the beginning. But, now the question is how to 
> do it in a correct way and retain current behavior (to not break callers)?
> 
> I see two options here:
> 1. Place the check of relocate_portio_handler() in 
> arch_hvm_ioreq_destroy() and make the later returning bool.
>      The "common" hvm_destroy_all_ioreq_servers() will check for the 
> return value and bail out if false.
> 2. Don't use relocate_portio_handler(), instead introduce a flag into 
> struct hvm_domain's ioreq_server sub-structure.
> 
> 
> Personally I don't like much the option 1 and option 2 is a little bit 
> overhead.

Well, 1 is what matches current behavior, so I'd advocate for you
not changing the abstract model. Or else, again, change the abstract
model in a separate prereq patch.

Jan